An overview of communication systems can be found in the white paper “Next-Generation Wireless Network Bandwidth and Capacity Enabled by Heterogeneous and Distributed Networks” by Freescale, which company also provides processors for such systems such as B4860 “QorIQ Qonverge B4860 Baseband Processor”. This processor targets macro cell base station designs for broadband wireless infrastructure and has four 64 bit, dual-threaded processor cores, six 16 bit 32GMAC/cycle calculation cores and baseband acceleration processing engines. It is designed to adapt to the rapidly changing and expanding standards of LTE (FDD and TDD), LTE-Advanced and WCDMA and supports different standards simultaneously. Technical data for the B4860 processor is available via http://www.freescale.com/webapp/sps/site/prod_summary.jsp?code=B4860.
As such, data processing of received communication signals in the base station is well known. An example of a communication system is a fourth generation (4G) cell based mobile communication system such as LTE system (Long Term Evolution, which is the most recent step forward from cellular 3G services) or the Wideband Code Division Multiple Access (W-CDMA) system.
In cell based mobile communication systems base stations are provided to communicate with the mobile devices. Such base stations usually have a layered structure. The basic data flow of an LTE base station will be described now.
FIG. 1 shows an example of a receiver for a communication signal in an LTE communication system. The Figure exemplifies a data flow in a base station on a signal layer, indicated as base station layer one BS Layer-1. The Figure shows a receiver 100 coupled to antenna data ANT 103 on a first input and a copy CPY 102 of the antenna data on a second input. In the Figures interfaces for transferring data are depicted by a gray background. The second input is coupled to a RACH unit for decoding the random access channel, which Random Access Channel (RACH) is used for initial access and when the User Equipment (UE) losses its uplink synchronization. The first input is coupled to a FFT unit 120 arranged for applying a Fast Fourier Transform (FFT) on the antenna data to provide post FFT data p-FFT 104. The post FFT data is coupled to a PUSCH decoder 121 for decoding the Physical Uplink Shared Channel (PUSCH), which carries the Layer-1 Uplink (UL) transport data together with control information. Supported modulation formats on the PUSCH are QPSK, 16QAM and depending on the user equipment category 64QAM. PUSCH is a channel, which uses SC-FDMA. The post FFT data is also coupled to a PUCCH decoder 122 for decoding the Physical Uplink Control Channel (PUCCH), which carries control information. Note that the Uplink control information comprises Downlink (DL) acknowledges as well as quality related reports. Finally, the post FFT data is also coupled to a SRS decoder 123 for decoding the sounding reference signals (SRS) used to estimate the uplink channel conditions in frequency division duplexing (FDD), and the uplink and downlink channels in time-division duplexing (TDD; since in TDD, uplink and downlink share the same spectrum) for each user to decide the best uplink scheduling.
SRS is a channel of the LTE standard and is a signal sent by the UE to the base station for channel quality measurements. Typical metrics are SNR (Signal-to-Noise Ratio) or SINR (Signal-to-Interference-plus-Noise Ratio) per resource block. SRS is defined in 3GPP specifications for Evolved Universal Terrestrial Radio Access (E-UTRA); Physical channels and modulation, Multiplexing and channel coding, and Physical layer procedures. Hence SRS is one of the LTE channels that need to be decoded by the base station. This processing requires several steps, large interface buffers in between each step, and frequent interaction between an FFT accelerator and a DSP Core. The current document further focuses on signal processing in the SRS decoder in the LTE base station, although such processing may also be applied in other decoders for decoding reference signals or other signal parts in a communication signal that use interleaved carrier frequencies.
FIG. 2 shows an example of an SRS decoder. The classic structure receives the post FFT data p-FFT 103 on the input of a de-interleaver DIL 210 to generate de-interleaved data p-DIL 201. The de-interleaving, usually called comb de-interleaving or DIL, is performed in the frequency domain on the respective part of a communication signal using multiple groups of interleaved carrier frequencies, e.g. SRS. A respective group of carrier frequencies is assigned to a specific communication channel, e.g. to a specific portable user device. An SRS comb refers to such a group of subcarriers, and may be called comb function or comb in this document. In LTE-SRS there are 2 combs which are interleaved (one subcarrier after another). Generally in LTE the 2 combs are assigned the even and odd subcarriers indexes, respectively.
Subsequently the de-interleaved data p-DIL is multiplied by the Zadoff-Chu sequence in ZC unit 220. The Zadoff-Chu sequence is a complex-valued mathematical sequence which, when applied to radio signals, gives rise to an electromagnetic signal of constant amplitude, whereby cyclically shifted versions of the sequence imposed on a signal result in zero correlation with one another at the receiver. Zadoff-Chu sequences are used in the 3GPP LTE Long Term Evolution air interface in the Primary Synchronization Signal (PSS), random access preamble (PRACH), uplink control channel (PUCCH), uplink traffic channel (PUSCH) and sounding reference signals (SRS). By assigning orthogonal Zadoff-Chu sequences to each LTE channel and multiplying their transmissions by their respective codes, the cross-correlation of simultaneous channel transmissions is reduced, thus reducing inter-cell interference and uniquely identifying channel transmissions.
The output of the ZC unit is processed by IDFT unit 230 to apply an inverse Discrete Fourier Transform, which provides an output of time domain signals TimDom 202 by conversion from frequency domain signals to time domain signals. The time domain signals may represent time domain channel taps, and are subsequently separated for a respective UE by UE-SEP unit 240. The separated time domain values from the UE-SEP unit are coupled to a discrete Fourier Transform unit DFT 250 to provide a frequency domain signal coupled to a noise estimator SNR-EST 260, which determines the signal to noise ratio for the respective UE based on the group of subcarriers assigned to that UE in the SRS signal so as to output the required SRS output for further use in the LTE base station, well known as such.
United States patent application US2012/0182857 provides an example of Sounding Reference Signal processing for LTE, and further elucidates the known processing elements, such as FFT, IDFT and ZC processing. A wireless communication receiver includes a serial to parallel converter receiving an radio frequency signal, a fast Fourier transform device connected to said serial to parallel converter converting NFFT corresponding serial signals into a frequency domain; a ZC root sequence unit generating a set of root sequence signals; an element-by-element multiply unit forming a set of products including a product of each of said frequency domain signals from said fast Fourier transform device and a corresponding root sequence signal, an NSRS-length IDFT unit performing a group cyclic-shift de-multiplexing of the products and a discrete Fourier transform unit converting connected cyclic shift de-multiplexing signals back to frequency-domain.
In practice, the process called “comb de-interleaving” (DIL) may be implemented by a DSP using multiple processing cycles, as well as requiring an intermediate interface to a memory to save temporary data. Such processing, memory and interface require processing circuitry and supply power.